introduction to temperature sensors isat 300 instrumentation and measurement 02/17/1999
TRANSCRIPT
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Introduction to Temperature Sensors
ISAT 300 Instrumentation and Measurement
02/17/1999
CoolBath
HotBath
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Temperature Measurement Devices
Electrical Effects Thermocouples Thermistors Resistance Temperature Detectors (RTD)
Mechanical Effects Liquid-in-glass thermometers
Radiation (non-contact) Pyrometers
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Introduction
Somehow, we need to translate the temperature of an object to a voltage that a data acquisition system can read.
A look at transducers that translate a change in temperature to a change in voltage or resistance.
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LanguageRange of Operation
Lowest to highest temperatureActive vs. Passive
Require external power source?Intrusive vs. Non-intrusive
Will this affect the system?Linearity
Is the conversion from voltage to temperature a line or a more complicated function?
Sensitivity
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Temperature TransducersThermocouples
Voltage Device, passive, non-linear -270 to 1372oC for type Ktype K
Thermistors Resistive Device, active, non-linear -100 to 300oC
RTDs (Resistance Temperature Detectors) Resistive Device, active, linear Large Range: -200 to +850oC for
Platinum Low sensitivity: 0.39 % per oC
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ThermocouplesTwo disimilar metals are joined togetherChange in temperature at junction
generates voltageSmaller size compared to thermistorsFast response time (time constant as small
as 1 ms)Ease of fabrication, long term stabilityLow sensitivity, small output voltageNeed reference temperature
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Thermocouples
Passive Device Voltage is developed as the function of the
the temperature of the junctionSeebeck Effect
A thermoelectric phenomenon The sum of two voltage effects, namely
Peltier effect and Thompson effectUse in Computer-Based Instrumentation
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Thermocouples - Background
Kirchhoff’s Voltage Law Sum of all voltages
in a loop = 0.
+_??V
+
_
2V
+_
_+
-3V
5V
Kirchhoff’s Current Law Sum of all currents
entering a node is 0.
5 mA
3 mA
?? mA
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How Does Thermocouple Work?Seebeck Effect
An electromotive force (EMF) exists in a loop made of two dissimilar metals when the two junctions are different temperatures.
T1 T2
A
B
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T1 T2
A
B
+
_+
_
AB BA
Peltier EMF Voltage at the junction of two dissimilar
metals Denoted as AB, BA
How Does Thermocouple Work?
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Thompson EMF Voltage in a wire caused by a temperature
difference between the ends. Denoted as A, B
T2
A
B
+ _
+ _
T1
A
B
How Does Thermocouple Work?
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The Ice Point - A Reference Junction
V V V V VT T T T DVM1 3 3 2 0
V V VT T DVM1 2 0
V V VDVM T T 1 2
B
T1
AT3
DVM
T3
Copper
A
+
+
+
T2ICE
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Using Thermocouples
Removing the Ice Point DVM shows the voltage due to the
difference between T1 and T2 Use the same equation as before!!
T2Chromel
Alumel
T1 DVM
T2
Copper V V VDVM T T 1 2
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Conversion Process in General Measure the Temperature, T2 Measure the voltage using the DVM, VDVM Convert the temperature of T2 to a voltage (table),
VT2 Add voltage from DVM to the voltage for T2, VDVM
+ VT2 Convert the voltage sum to T1 (use a table)
T2Chromel
Alumel
T1
T2
CopperDVM
Using Thermocouples
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Example: A type R thermocouple system with an ice reference has an output of 9.1 mV. What is the temperature of the sensing junction?
From OMEGA Type R Thermocouple Reference Table, ttp://www.omega.com/techref/tctables/rc-2.html9.090 mV corresponds to 891oC and 9.103 mV corresponds to 892oC.
Linear interpolation gives a temperature 891.8oC for 9.1 mV.
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Thermocouple Reference Tablewww.omega.com/techref/tctables/rc-2.html